One or more brushless DC cooling fans are conventionally employed in power conversion systems to remove heat generated by semiconductor switching devices, magnetic energy storage components and filtering components, as well as other circuit components that are part of the power conversion equipment. The cooling fans advantageously allow the power conversion equipment to be operated in an extended temperature range. Cooling fans thus play an important role in overall system reliability and lifetime.
In most applications, high cost brushless fans having integral speed sensors (sometimes referred to as "tachometers") are usually employed to cool the electronic power conversion equipment. Typical speed sensors transmit a speed feedback signal in the form of a fixed duty cycle pulse train to a detection circuit via a third terminal on the fan. The detection circuit receives and interprets the pulse train, governing a fan alarm and system safety interlock as a function thereof. Whenever the detection circuit does not detect a speed feedback signal, the detection circuit trips the alarm and the system safety interlock to warn of a potentially harmful situation and power down the power conversion system to protect electronic components in the power conversion system from overheating.
Unfortunately, fans with integral speed sensors are more expensive and suffer reliability problems. They are costly to manufacture, test and trouble-shoot. Any problems with the speed sensor, fan connectors, wiring, cables or detection circuitry may cause the detection circuit to shut down the power conversion system, even when the fan itself is operating normally. To avoid the expense and unreliability of fans with integral speed sensors, fans without such sensors are being increasingly employed, thereby risking undetected fan failure.
It should be apparent that the risk of a spurious shutdown of a power conversion system employed in a critical application, such as telecommunications, is unacceptable. It should be equally apparent, however, that legitimate fan failure should be cause for taking protective measures. Accordingly, what is needed in the art is a method and circuit for detecting the operational state of a fan, without requiring an integral speed sensor.